Convert package to ESM

[nzakas]

To convert this package from CommonJS to ECMAScript modules (ESM):

• Add type: module to package.json
• Convert files in lib to ESM
• Convert files in tests/lib to ESM
• Upgrade Mocha
• Create build script in package.json to generate CommonJS file (.cjs) in dist directory (use Rollup)
• Add dist to .gitignore
• Run build in prepare script in package.json to ensure build is run at key moments
• Update package.json so that main points in the CJS file in dist and exports provides both require and import options
• Update README instructions